1. Field of the Invention
The present invention relates to conical spiral antennas for use in radio wave communications.
2. Prior Art
Radio wave communication antennas for mounting on vehicles, such as boats and automobiles, usually are vertical monopoles with a substantially omnidirectional radiation pattern. They must be sturdy so as to withstand the vibrational and accelerational forces applied to them through the vehicle. Preferably they also are light and compact for easy mounting on the vehicle. Monopoles with a broad band characteristic have the advantage of receiving or transmitting radio waves of a fairly broad frequency range.
Conical spiral antennas can have all of the features discussed above and have been utilized in several mobile applications, such as for receiving and transmitting radio waves in the "citizens band" frequency range which is in the neighborhood of 27 megahertz. Typically a small diameter, conically spirally wound antenna conductor wire forms a slightly tapered cone ranging from 1 or 2 feet (0.3 or 0.6 meters) to several feet in height. Particularly for mobile use, the conductor wire must be supported to maintain its shape. In the past such a conductor wire has been sandwiched between a substantially frustoconical inner core and a substantially frustoconical outer protective casing, one or both of which can be plastic material. In manufacture of such an antenna, the preformed inner core is used as a mandrel around which the conductor wire is wound with a desired axial spacing of adjacent spiral convolutions; and the wound core is fitted inside the protective casing. It can be difficult to wind the conductor wire precisely on the inner core, and the resulting antenna may be unacceptably heavy due to the combined weights of the core, the wire and the outer casing.